The present invention relates to the magnetic resonance art. It finds particular application in conjunction with medical diagnostic magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with magnetic resonance spectroscopy systems and other applications which require RF and gradient magnetic field sequences.
Heretofore, magnetic resonance imagers have included a superconducting or resistive magnet which generated a temporally constant primary magnetic field. A cylindrical bore extended along a central axis of the magnet such that the primary magnetic field was generated longitudinally along the bore. Gradient magnetic field coils for generating magnetic field gradients across the primary magnetic field were mounted along or as part of the cylindrical bore. A series of gradient amplifiers were connected with the gradient coils for producing high amperage current pulses to drive the gradient coils.
A radio frequency whole body coil was commonly placed within the bore. A digital transmitter was connected with the radio frequency coil to drive the coil with radio frequency pulses for inducing and manipulating magnetic resonance of selected dipoles of a subject within the cylinder. A radio frequency synthesizer was connected with the digital transmitter for providing selected RF waveforms thereto. Magnetic resonance signals emanating from the resonating dipoles of the subject in the bore were received by the whole body RF coil or surface coils and conveyed to a digital receiver.
In order to implement the many available imaging sequences, a general purpose computer was commonly used to control the current amplifiers for the gradient coils, the radio frequency synthesizer, the digital transmitter, and the digital receiver. The general purpose computer also received the magnetic resonance signals from the digital receiver and reconstructed appropriate electronic image representations therefrom. Commonly, the general purpose computer had several analog and digital input and output channels to perform these multiple tasks. More often, the general purpose computer was combined with a timing device which generated timing and sequencing signals which, in turn, were connected to the several analog and digital input and output channels.
Conventional imaging sequences required a large number of precisely clocked signals in a relatively short time interval. This placed very tight timing tolerances on the general purpose computer. In many applications, the timing tolerances were beyond that which could readily be achieved with a moderately priced general computer. When used to generate timing and synchronization for multiple channels, a sequencer device greatly reduced the timing requirements expected of the main computer.
By using a sequencer device, the details of the waveforms at each timing interval could be fed to the gradient amplifiers and the RF synthesizer with rapid, precise timing. The total load on the main computer could be further reduced if the sequencer was preloaded with a series of timing patterns and waveforms relieving the general purpose computer of providing timing pattern and waveform information to the sequencer device in every clock cycle.
Various techniques have been provided for reducing the timing load on the main scan computer. For example, U.S. Pat. No. 4,713,615 of Barratt, U.S. Pat. No. 4,743,851 of Lim, and U.S. Pat. No. 4,710,716 of Keren, describe obliquing techniques which reduce the load on the main computer when the selected slice is angled or obliqued. U.S. Pat. No. 4,845,613 of Netter describes a main sequencer or master microcode which controls several channels. U.S. Pat. No. 4,707,661 of Hoenninger describes a main sequencer microcode which accomplishes long repetitive patterns with less memory by linking pieces which are executed multiple times.
U.S. Pat. No. 5,144,242 of Zielenga provides a detailed example of a sequencer with a specific provision such that the central processing unit can update the program in the sequencer while the sequencer is in operation.
U.S. Pat. No. 4,761,612 of Holland and 4,928,063 of Lampman illustrate dedicated correction circuitry for cancelling eddy current effects.
The present invention relates to techniques and hardware which relaxes the timing requirements and total loading on the main computer through scan sequence automation.